Add internal_reviewers field to group

[uesteibar]

It would be great to have an internal_reviewers group setting, that would set how many reviews are required from that group if the PR author belongs to it. Example:

For

groups:
  - name: sample_group
    reviewers: 2
    internal_reviewers: 1
    usernames:
      - user_a
      - user_b
      - user_c

• If user_a opens a PR, then only one reviewer (either user_b or user_c) will be assigned
• If user_d opens a PR, then two reviewers from sample_group will be assigned

If reviewers is not added, then we should only set reviewers when a user within the team creates the PR.